23. Choosing the Primary and Secondary interleaving pattern
may entirely bamboozle many readers
or designers without any experience of winding or analysing audio
frequency transformers where
bandwidth from 14Hz to at least 70kHz is required. To achieve wide
bandwidth the primary and
secondary winding sections must be interleaved, ie, alternately
and evenly sequenced as the bobbin
is wound to its full height, so that the primary winding is
effectively placed close to the secondary
winding for the length of the windings. Such interleaving can
achieve a flat HF response which may
exceed 200kHz. 200kHz is rarely ever seen in most OPTs in mass
produced amps because the
interleaving is minimal, say 3 Pri sections with 2 Sec sections.
But where there may be 5 Pri sections and 6 Sec sections, the HF
response can extend to 270kHz.

There are NO FORMULAS which allow a designer or a DIYer to enter
all parameters known for any
OPT to get one of many possible interleaving patterns.

I have prepared Tables 2, 3,4,5, below to allow the designer to
select a possible interleaving pattern
based on the maximum power rating for the OPT, and number of P
winding layers.
For OPT-1A, Power = 75W, 16 P layers. See Table 4.

4S + 5P has layer sequence in bobbin = 2p - S - 4p - S - 4p - S -
4p - S - 2p where each "p" is a
primary layer. "2p" is a Primary Section with two layers. "4p" is
another Primary Section with 4 layers.
S is a Secondary Section, usually with one layer of wire much
thicker than each primary layer.
The secondary sections may be sub-divided into 2, 3, or 4 windings
to allow connections with other
secondary sections to gain load matches for 4r0, 8r0, 16r0 with
equal leakage inductance and HF
response, equal winding loss % and ac current density equal in
each winding, and while using all
secondary turns available.

The unequal number of P layers in each primary section occur to
get best HF response with
leakage inductance occurring evenly and symmetrically distributed
throughout the height of winding.
Where the first and last winding section wound on is for Primary,
these sections should have
approximately 1/2 the layers of the inner primary sections. The
inner P sections usually have equal
layers, but the number of layers may vary +/-33%. If there were a
total of 18 P layers, inner P
sections have 4p and 5p, and the S sections cannot have equal
layers.

All Push Pull OPT designed here using my methods should display
adequate magnetic coupling
between the two halves of primary winding because each half is
well coupled to the secondary.
This avoids a serious problem with distortion due to current cut
off in tubes in class AB. ( This was
a problem in 1930's before anyone knew how to interleave P & S
which cost more to do properly. )

As the number of P+S sections are increased, the leakage
inductance reduces, with minimum LL
where there are equal number of layers for P and S windings, each
interleaved S-p-S-p-S-p-S-p-S etc.
While having low LL is good to obtain good HF response, the shunt
capacitance increases with the
many P to S interfaces, and the number of insulation layers
prevents the required amount of copper
in the bobbin to get enough turns. The tables 2,3,4,5 offer
interleaving patterns which reduce LL
adequately, and keep shunt C adequately low so that resonance
between LL and shunt C occurs
well above 70kHz where resonance can be damped by R+C Zobel
networks without affecting the
amplifier performance below 20 kHz. For lower Primary RL and
higher amplifier power the OPT
becomes larger and interleaved P and S sections are increased
because LL must be made low to
suit lower loads. With lower loads the capacitance has less
effect. So a small 15W OPT may only
need 3S + 2P sections for 70kHz, but a 400W OPT may need 6S + 5P
sections.

24. Choose insulation thicknesses from Table 6.
0.05mm insulation is used between primary layers to lessen
possibility of shorted turns even where
the layers have similar Vac and Vdc, and to facilitate
winding with small diameter wire.

Where there is large Vdc or Vac difference between any wire layers
or between wire and core
the insulation should be thick enough to avoid insulation
breakdown and arcing. Most modern
plastic film sheets like polyester have high enough voltage
breakdown ratings to prevent any
problem in any OPT with Vdc up to +1,500V and Vac adding to peak
volts across insulation
up to 3,000V.
Insulation is not chosen only for its breakdown voltage rating but
for its thickness to reduce
shunt capacitance.

I see no reason why Nomex insulation could not be used for OPTs.
The data for Nomex 401 electrical grade paper is here :-
http://www.dupont.com/content/dam/assets/products-and-services/electronic-electrical-materials/assets/DPP_Nomex410_K20612-1.pdf

From what the Nomex table shows, V ratings for thicknesses may be
tabled :- Table 7.

Nomex
insulation

Volts / 0.025mm
rating

Volts for
thickness

0.05mm

460

920V

0.1mm

525

2,100V

0.18mm

865

6,228V

0.25mm

845

8,450V

0.3mm

870

10,440V

0.38mm

850

12,920V

0.51mm

810

16,524V

0.61mm

810

19,764V

0.76mm

680

20,672V

The Nomex table information does not show a linear increase in
Volts / mil for thickness but
there should be no worry about insulation thickness not having
high enough V ratings
for any OPTs designed by methods at this website.

Before 2017, the insulation table at output-trans-PP-calc-5.html
for minimum insulation
thickness was used for all designs at this website. The table
lists much thicker insulation
than in the Nomex table for Volts. But this meant Shunt C was
minimised.

NOTE. Insulation may be thicker than the minimum selected
to achieve lower shunt C
to extend HF response and reduce phase shift for above 10kHz to
make stabilisation with
NFB easier and more reliable, especially where RLa-a is between
10k0 and 30k0.

NOTE. With no secondary load present, and high signal input to amp
with pure tetrode or
pentode mode the total Vdc + Vac peak swing may exceed +/- 2,000V
at each anode due to
energy stored in leakage inductance with no R load to shunt it, or
without clamping diodes
between each anode and 0V to limit negative going Vpk to 0V, and
hence limit positive going
Va peak to twice Ea.
However, arcing is unlikely unless excessive Vac is maintained for
a long time, or there
moisture or pollution present or if poor insulation material is
used.
I have seen old OPTs which used plain waxed paper insulation and
some arced between
wire leads to windings with less than 350Vdc, because of
accumulated grime and moisture
and cracked enamel where wire was bent. In one case I was able to
clean away the dirt,
dry it out, and paint on electrical vanish and it never arced
again. But arcing can occur inside
a poorly insulated OPT and nobody can repair it.
Varnishing should improve the insulation resistance. Insulation
should be polyester or Nomex.

I do not favour Kraft electrical paper. Its dielectric constant
may be like Nomex but afaik, the
dielectric constant increases when paper is soaked with varnish
and baked, making shunt C
higher. For PTs which operate at 50Hz, the shunt C is no problem
and electrical grade paper
and cardboard or "elephantide" is fine.

25. Draw basic interleaving pattern.Fig 1. Basic interleaving.
Fig 1 shows basic interleaving pattern for bobbin winding layers
chosen so far, and the
diagram represents the build up of concentric layers from the
bottom of a bobbin, with
traversing direction alternating for every primary layer. All
secondaries are wound with
the same traverse direction.

Primary layers begin at Anode 2, and winding layers traverse
across bobbin as arrows show.
The bottom primary section has two layers with entry and exit
wires brought out 150mm to a
temporary hold point on lathe chuck, and each wire is labelled
with masking tape and a number.
Secondary entry and exit wires are 150mm long and labelled with
tape with letters. Varnish
which dries slowly is brushed on to all wire layer and and
insulation layer surfaces during
winding if there is no way to have windings varnished with vacuum
impregnation after winding
is complete.

Your first drawing of the contents in a bobbin may be a much less
tidy pencil sketch on a sheet in
a notebook, but it must tidy to avoid confusion or a mistake.

For OPT-1A :-
For Pri to Pri, with consecutive anode layers where V difference
< 100V, shunt C is not a
problem use 9 x 0.05mm insulation which will make winding easier.

For Pri to Pri, with anode and cathode layers with Vdc + Vac
difference = 1,000V, use 0.5mm
or 0.38mm if total winding height is found to be too high. Allow
for 2 insulation layers x 0.5mm.

NOTE. These sec turns per layer are for the thickest wire
possible. Sec layers should
be have any gaps between turns. 45t will fill the layer almost
fully. Thicker sec wire
with less turns per layer should not be used if the height of all
bobbin content exceeds
0.8 x Core H which may prevent insertion of core Es to a wound
bobbin.

But Sec turns per layer may be increased up to +20% with smaller
wire dia without
causing much increase of winding resistance to get a match to a
higher load.
If 45t are a match for 3r0, then +20% = 54t which matches 4r3. But
selection of smaller
wire size always reduces total secondary copper in bobbin and Sec
Rw loss %
always increases.

Most of the maximum power delivered to 4k0 is class AB.
Where the calculated RwP loss % is nearly equal RwS loss % for
class A operation, and for
where the OPT is used for maximum class AB operation where little
class A Po is produced,
and where RLa-a = 4k0 for Po > 70W class AB, the effective
total Rw losses increase by factor
of 1.4.

Therefore, total loss for over 70W in class AB = 1.4 x 6.31% =
8.8%, and less than 10% which
is considered OK for when the amp is working as hard as it ever is
expected.

Selection of Secondary sub-sections for range of wasteless load
matches.

Fig 6. For 6 Sec Layers.36 continued....Choose 4A from Fig 4, with 4 sec
layers sections, with top layer section
divided into 3 sub-sections.
NOTE. A range of loads close to wanted 4r0, 8r0 and 16r0 is only
possible if the total sec
turns are exactly divisible by 12, and turns in each layer are
exactly divisible by 3.
For 51t per layer, it cab make 3 x 17t windings, and 4 layers
gives 204t exactly divisible by 12.

Examine Sec sub-section pattern 4A. Fig 7. 4A sub-sections.
Fig 7 shows 4A pattern says "Total turns divisible by 12" so turns
in each of 4 layers will
be exactly divisible by 3, or else the pattern will just not work
properly.

Pattern 4A gives THREE useful load matches to RLa-a 8k0. Each load
match covers the load
variation for the average or nominal Z found in many manufactured
speakers. For example,
a "4r0" speaker may have minimum Z = 2r5 and maximum 20r0. When
used with sec = 51t,
RLa-a load varies between 5k2 and 41k4. The amp will handle the
load change OK.
Giving the tubes the best load match right minimizes the bad
effects of speaker Z variation, ie,
minimizes THD and IMD. But with only 3 possible load matches, if
someone wanted only pure
class A operation with 4r0 speakers, they would be disappointed
because the ratio of 8k0 : 3r9
is the highest available, and to get the ratio should be about
16k0 : 3r9, and this means Ns
would have to be lower and about 0.71 x 51t.
The other disadvantage may be that there is no perfect way to get
70W class AB for 16r0
speakers. We would want OPT ZR = 4k0 : 16, which means TR = 15.8 :
1, so Ns would
have to be 147t. However, with 4 sec windings of 51t each, the
central 2 could be paralleled
and then connected in series to each of the other 2 x 51t to give
Ns = 143t, which gives load
ratio = 4k0 : 17r4, so that high Po would be possible for 16r0
speakers. But I have NEVER
known anyone wanting high Po for 16r0 speakers so being able to
strap the OPT for high Po
to 16r0 or even to include provision for 16r0 load match is
probably never ever going to be
needed.

Fig 8. 4A strapping pattern.
The strapping pattern for 16r0 is not included because it is
unlikely to be used, but feel
welcome to figure out the strapping for 16r0.

Examine Sec sub-section pattern 4C.

38. Let us consider using pattern 4C from Fig 4,Fig 9. 4C sub sections.
Fig 9 Pattern 4C gives six useful load matches with 51t per layer
with each subdivided to 17t + 34t.
5 load matches have equal current density with one for Ns = 153t
with unequal current density.
All will work OK.

There are plenty of good load matches for everyone !

Table 10.

Np = 2,320t,
RLa-a

Ns =
6 //34t
ZR 4,656

Ns =
4 // 51t
ZR 2,069

Ns =
3 // 68t
ZR 1,164

Ns =
2 // 102t
ZR 517

NS = 3 x 51t
= 153t,
ZR 230

NS = 4 x 51t
= 204t,
ZR 129.3

Class AB
power

Class A
power

4k0

0r86

1r9

3r4**

7r7****

17r4

30r9

75W

5W

6k0

1r3

2r9

5r1

11r6

26r0

46r4

60W

7.5W

8k0

1r7

3r9**

6r9****

15r4

43r8

61r9

50W

10W

12k0

2r6

5r8

10r3

23r2

52r2

92r8

35W

15W

16k0

3r4**

7r8****

13r7

30r9

69r6

123r7

24W

20W

24k0

5r1

11r6

20r6

46r4

104r3

185r6

nil

18W

Table 10 gives 5 useful matches allowing over 70W to any sec load
between 0.9r and 185r,
with Ns = 34t, 51t, 68t 102t and 153t.
The Ns = 153t has 2 central 51t in parallel, and is in series with
2 x 51t, and allows high class
AB Po for 16r0 speakers. The Ns = 204t has 4 x 51t sec layers in
series, and would suit
powering a line array speaker with many drivers in series, or it
could power a 100V line to
deliver about 30W to a number of remote speakers fitted with step
down transformers.

NOTE. The pattern 4C from Fig 4 will require 16 terminals for the
secondary windings to be
set out on a terminal board to allow soldered wire links of the
terminals in 4 different patterns
to achieve the desired load matching. Many audiophiles would be
confused with making a
load match change so they would need a technician to make sure the
load match change is
done correctly. If a mistake is made with strapping terminals
differently, it could damage the
amp, or give very poor music.

Fig 10. 4C board terminals.
Fig 9 is terminal board for 4C link patterns for nominal 8k0 :
2r0, 4r0 and 8r0.
The real loads with RLa-a 8k0 are in fact for 1r7, 3r9, 6r9, but
this allows for the minimum Z
of any speaker being lower than its nominal value. These
selections will suit those wanting the
most pure class A. There is no strapping pattern for 16r0 speakers
because they are unlikely
to be used, but will work OK with "8r0" outlet.